210 MICRO-ORGANISMS IN WATER 



secured in the case of copperas when lime is also 

 added. 



In the following table are finally recorded the results 

 of experiments made to ascertain the relative efficiency 

 of the several chemicals in purifying one and the same 

 sample of sewage. In this series the most advantageous 

 proportion of lime was employed, and the other chemi- 

 cals were applied under the conditions found in previous 

 experiments to be the most favourable, whilst the actual 

 amounts used were so selected as to be of the same 

 pecuniary value as that of the lime. 



Eesults of treatment of Sewage with equal money-values of 

 different Chemicals 



Cost of , 



chemi- Number of 



cals in bacteria 



cents. 1 m 1 (J - ( '- 



Reduc- 

 tion pei- 

 cent. 



Original sewage i \ 25,840 



Ditto after settling 1 hour without additions . 10,920 



Effluent with 1,800 Ibs. of lime per million 



gallons of sewage 30 1,911 93 



Effluent with 1,000 Ibs.of copperas and 700 Ibs. 



of lime per million gallons of sewage . 30 j 16,044 ' 38 

 Effluent with 270 Ibs.of ferric oxide per million 



gallons of sewage 30 2,047 92 



Effluent with 650 Ibs. of alum per million 



gallons of sewage 30 2,475 91 



Effluent with 360 Ibs. of ferric oxide per 



million gallons of sewage . . .40 1,980 93 



Effluent with 870 Ibs. of alum per million 



gallons of sewage 40 1,800 93 



1 Per inhabitant annually, calculated on the assumption that 1UO gallons of sewage daily arc 

 produced by each individual. It should be noted that American sewage is generally very much 

 more dilute than that to which we are accustomed in British towns. 



In connection with these interesting results obtained 

 by the Massachusetts Board, we may appropriately 

 append the following general conclusions from their 

 Eeport of 1890, p. 788 : ' When a nuisance is produced 

 by sewage in any way, the direct cause is usually the 

 development of organisms fed by the organic matter 

 and nitrogen compounds of the sewage. To secure the 



